Basal body temperature (BBT) is your body's temperature when you're fully at rest. Whether you're charting your BBT in order to get pregnant or as a method of birth control, getting accurate readings is important, and it's not as easy as you might think.

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Steps

1

Use a thermometer that reads to the tenths place (ie: xx.x) a digital thermometer is ideal. You don't need a specifically designed thermometer for taking BBT readings. Do not use a standard (mercury) thermometer, which isn't specific enough.[1]

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2

Get enough sleep and, if possible, regular sleep. Irregular sleep patterns (including less than three hours of uninterrupted sleep) can result in false readings.[2]

3

Take your temperature every day at the same time, before getting out of bed.[3] Set an alarm and keep the thermometer at your bedside or under your pillow. Don't stand up, walk around, eat anything, drink anything, or engage in any kind of activity (even shaking a mercury thermometer) until after you've taken your BBT (which is supposed to be taken at full rest).[4]

4

Make a graph on graph paper or on the computer, with dates on the bottom and basal temperature on the side. You can find and print fertility charts online, as well as sign up for a fertility charting service.

5

Look for a gradual or sudden rise in temperature (between 0.5 and 1.6 degrees F).[5] Fertility is highest during the two to three days before your basal temperature rises[6] so if you can observe any month-to-month patterns in when your temperature rises, you can predict the best time to conceive (or the best time to abstain if you're using this method for birth control).

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The time you take your basal temperature should be as early as possible, since you can always go back to sleep afterwards if you have an irregular sleep schedule.

For better results, observe changes in cervical mucous as well. At the beginning of a menstrual cycle, it's sparse, tacky, and dense; when ovulation is near, it starts becoming more slippery and plentiful, resembling raw egg white. Checking cervical mucous can reinforce observations about ovulation gathered by charting BBT.

Warnings

Avoiding intercourse during your fertile days will likely reduce your chances of pregnancy, but it will not in any way prevent the spread of sexually transmitted infections.

Charting BBT is not 100% reliable as a birth control method and is best used in combination with other birth control methods.

A rise in BBT can also be caused by emotional disturbance, stress, a cold or infection, jet lag, drinking alcohol the night before, or using an electric blanket.[7]